I did a brief literature search and found nothing to support the cows milk/mucus production idea. The only controlled blinded study I found showed no change from drinking cows milk vs soy.
I think this idea also gets taken too far. Last fall while visiting my sister, (and attending an AMB conference, no thats not atificial baby milk) my baby developed croup bad enough for me to call my sister's pediatrician in the early AM. The triage nurse I first spoke to advised warm juice over breastmilk because of mucus production!
I've also run this idea by a peds gastroenterologist/nutritionist and a peds allergist. Either support this idea.
To borrow a line from Jerry Maguire, "Show me the money" or at least the studies. I dont buy it.
-Rob